As used herein, multiphase fluid means a fluid composed of both liquid and gas phases and which may also contain solid particles.
The major challenge of pumping multiphase fluids is to pump the fluid through a flowline under a wide range of conditions. As used herein, flowline includes flowlines from producing wells, as well as any other conduit such as pipeline or vessel. Single phase pumps and compressors are designed to pump only fluids of constant density, or single phase. When more than one fluid phase is present, such as gas and liquid, the fluid densities vary drastically. The constantly varying densities combined with the high speed of the moving components in the pumping equipment generate changing and unbalanced forces that the single phase pump and compressor are not designed to handle.
Currently, the most widely used method of pumping a multiphase fluid is to separate the fluid into its various components upstream of the pump; then pumping these components with separate pumps through separate flowlines. This approach allows the existing equipment to continue to pump single phase fluids. However, this separation method and separate pumping is expensive and may require elaborate equipment.
Multiphase pumps make it possible to directly pump multiphase fluids through pipelines, thereby eliminating the need to separate the fluid into discrete phases before pumping. Two types of pumps, rotodynamic and positive displacement pumps, are known for multiphase pumping. Rotodynamic pumps, which include centrifugal and axial turbine types, are well-suited to low pressures. However, rotodynamic pumps have great difficulty pumping multiphase fluids containing even a small percentage, say 2% to 4%, of gas phase (as opposed to dissolved gas).
Positive displacement pumps can more easily pump gas/liquid mixtures so they are more suitable to pump fluids with higher gas fractions. They can also achieve high discharge pressures. However, a drawback of positive displacement pumps is that they work with small clearances and therefore are more susceptible to sand damage and erosion. Most positive displacement pumps are screw type pumps. An elongated screw is housed and rotated on a cylindrical barrel. As the screw rotates, it pulls in fluid from a first end of the barrel and ejects it under pressure from the other end.
In the April 1988 issue of Offshore magazine, an article entitled "The WST-Pump, A Reciprocating Rotary Ram Pump for Multiphase Boosting" disclosed a pump/compressor that pumps multiphase fluids using a piston type pump. The WST-Pump employs an annular array of multiple cylinders that rotate around an axis as pistons on the cylinders alternately intake and discharge multiphase fluid.